A CALPHAD assessment of the Cu-Pt system has been carried out. Two and four sublattice models were applied to describe the Gibbs free energies of ordered phases where the contribution of SRO is taken explicitly into account through the reciprocal parameters. The disordered fcc A1 and liquid phases were treated as substitutional solutions. A consistent set of parameters for the phases in the Cu-Pt system as obtained, and those parameters can satisfactorily reproduce the experimental phase equilibria and thermodynamic properties, such as enthalpies, activity of Cu, and long-range order parameters.

A thermodynamic description of the intermetallic compounds in the Zr-Sn binary system has been obtained using total energy calculations by means of the Vienna ab initio simulation package. Our calculations show that hexagonal compounds Zr₅Sn₄ and Zr₅Sn₃ are the most stable phases in the Zr-Sn binary system. Their high stability is found to be due to hybridization of the Sn 5p with Zr 4d electronic states. Based on the calculated energies, the conclusion is made that Zr substitution on the Sri sites takes place in the Zr₄Sn phase, which accounts for the unusual stoichiometry of this Cr₃Si structure type compound.

We describe current approaches to thermodynamic modelling of liquids for the CALPHAD method, the use of available experimental methods and results in this type of modelling, and considerations in the use of atomic-scale simulation methods to inform a CALPHAD approach. We begin with an overview of the formalism currently used in CALPHAD to describe the temperature dependence of the liquid Gibbs free energy and outline opportunities for improvement by reviewing the current physical understanding of the liquid. Brief descriptions of experimental methods for extracting high-temperature data on liquids and the preparation of undercooled liquid samples are presented. Properties of a well-determined substance, B-2 O-3, including the glass transition, are then discussed in detail to emphasize specific modelling requirements for the liquid. We then examine the two-state model proposed for CALPHAD in detail and compare results with experiment and theory, where available. We further examine the contributions of atomic-scale methods to the understanding of liquids and their potential for supplementing available data. We discuss molecular dynamics (MD) and Monte Carlo methods that employ atomic interactions from classical interatomic potentials, as well as contributions from ab initio MD. We conclude with a summary of our findings.

Developing the third generation of Calphad databases with more physical basis valid within a wider temperature range is the aim of the present work. Atomistic scale (ab-initio) methods, particularly techniques based on DFT theory, are used for modelling dierent phenomena, so as to gauge the capacity for use in Calphad modelling. Several systems are investigated in this work for studying dierent phenomena, such as magnetism and vibration of atoms. In the case of pure elements (unaries), thermodynamic properties of Mn, Al and C are optimized in the whole temperature range by the help of new models. In addition, DFT results and specic characteristics of these elements are also used to develop models for describing magnetic properties and atomic vibrations. With regards to coupling between DFT and Calphad, the EMTO technique is used for determining the magnetic ground state of the metastable hcp phase in Fe and Mn, and the TU-TILD technique is used for modelling solid phases above the melting point. TU-TILD is also used for calculating thermodynamic properties of bcc Mn at nite temperatures. The same phenomena are investigated in higher-order systems, i.e. the binaries Fe-Mn and Mn-C. Thermodynamic properties and phase diagrams of these systems are assessed against experimental data. Moreover, the revised magnetic model is used for modelling magnetic properties in these systems.

It is shown through this investigation that although the DFT methods are powerful tools for model development and for resolving discrepancies between dierent experimental datasets, they should not be overly-trusted. Caution must be taken when using DFT results, since the approximations and assumptions for computational implementations may cause some errors in the results. Moreover, implementing them into Calphad software as a connected methodology is not currently accessible due to the computational limitations.

It is concluded that coupling between the DFT and Calphad approaches can currently be achieved by using DFT results as an input in Calphad modelling. This will help to improve them until they can be integrated into the Calphad approach by the progress of computational possibilities.

One of the advantages of developing the third generation Calphad databases is the possibility of using the 0 K DFT results in Calphad modelling, since the new databases are valid down to 0 K. This has not been possible in the past, and such potential opens a new door to bring more physics into the Calphad approach.

In developing the third generation of Calphad databases, density-functional-theory methods are particularly important for validating physical models and for providing input data where experiment is unavailable. Since Cal-phad modelling needs data at finite temperatures additional sampling tech-niques are required, and the recently developed TU-TILD method is one such technique that allows for an efficient and accurate calculation of free energies. It is used in the present work for the complicated paramagnetic bcc phase of Mn. Our results show a good agreement with experimental data, validating that this method is well suited for calculating thermody-namic properties of magnetically complex and dynamically unstable phases as needed for Calphad modelling. It is also shown that a transition from bcc to the ω phase occurs below 1200 K.

In connection to developing the third generation of Calphad databases a new thermodynamic description is presented for unary carbon. Models used in this work have more physical basis and are valid down to 0 K. The anisotropy in graphite, caused by weak Van der Waals inter-plane forces makes it difficult to fit the heat capacity data by a single Einstein tempera-ture for modelling the harmonic vibration of the atoms. By using multiple Einstein temperatures this problem is solved and a good agreement with the experimental data at low temperatures is achieved. Diamond is mod-eled using new models too, and the two-state model is used for modelling the liquid phase.

In developing the third generation of Calphad databases, after having proper models for the unaries, the next step is to assess higher-order systems, i.e. binaries, ternaries etc. A new description for the Fe-Mn system is presented in this work, based on the Calphad approach. New models with a stronger physical basis are used to model Gibbs energy of the phases. For this purpose, the revised magnetic model is used to fit the magnetic properties versus the most recent experimental and DFT data. An acceptable magnetic phase diagram is reproduced, which results in a more reasonable fit for the phase diagram and will prevent possible artefacts in higher-order systems. The descriptions are valid down to 0 K, which make them very useful as an input for modelling phase transformations occurring at low temperatures.  

 In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. Harmonic vibrations of atoms are modelled using the Einstein temperature; anharmonic vibrations, electronic and magnetic contributions to the solid phases are represented by specific terms. The two-state model is used for the liquid phase. To continue the development, a new description for unary aluminum is presented in this work. In particular, finite temperature density-functional-theory (DFT) results are used to discuss and suggest the most applicable and physically based model for Calphad assessments of solid phases above the melting point. 

Accurate values needed for the most commonly used indicators of good Glass Forming Ability (GFA) in alloys, i.e. the liquidus (T-l), crystallization (T-x) and glass transition (T-g) temperatures, are only available after successful production of the metallic glass of interest. This has traditionally made discovery of new metallic glasses an expensive and tedious procedure, based on trial-and-error methodology. The present study aims at testing the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) approach for predicting GFA in the Fe-Nb-B system by the use of the Thermo-Calc software and the thermodynamic database TCFE7. The melting temperatures and phase stabilities were calculated and combined with data for an atomic size mismatch factor, lambda, in order to identify and map potential high-GFA regions. Selected compositions in the identified regions were later produced by suction casting and melt spinning, and the potential success verified using X-Ray Diffraction (XRD). Differential Scanning Calorimetry (DSC) was also used to compare thermodynamic calculations for the model predictions, and evaluate standard GFA indicators. The model is found to fit well with literature data, as well as predict new bulk glassy compositions at and around Fe70.5Nb7B22.5. These results show promise in further predictive use of the model.

Titanium alloys are highly sought after due to their excellent mechanical properties. One of the most commonly used Ti alloys is Ti-6Al-4V, which contains 6% Al and 4% V by weight. Despite the popularity of this alloy, no thermodynamic description of the ternary Al-Ti-V system has been published in the open literature. In this work an assessment procedure of the ternary Al-Ti-V system was initiated based on the binary descriptions by Witusiewitcz et al. (J. Alloys Compds. 465 (2008) 64-77 [1]) for (Al-Ti), Gong et al. (Int. J. Mater. Res. 95 (2004) 978-986 [2]) for (Al-V) and Saunders (COST 507, 2 (1998) 297-298 [3]) for (Ti-V). When combining the three binary systems and looking at the extrapolated ternary isothermal sections, it was found that there was a very large miscibility gap in the bcc phase. The origin of this miscibility gap was mainly the Al-V system and therefore it was decided to reassess this system. The Al-V system was reassessed according to available experimental data along with the enthalpies of formation of all compounds as well as the enthalpies of mixing for all terminal phases obtained by first-principles calculations based on the density functional theory. For the Al8V5 phase there are two different sets of data for the enthalpies of formation. These two sets are investigated in this work and it is found that the set not used by Gong et al. in their assessment of the Al-V binary system gives better extrapolations. The final description produced improved extrapolated ternary isothermal sections.

The so-called TWinning Induced Plasticity (TWIP) steels have gained a lot of attention in the last couple of years due to their excellent mechanical properties; they show very high strength and are at the same time very ductile. The TWIP steels are austenitic and form mechanical twins under deformation. All TWIP steels have very high manganese content and a certain sub-group of these steels, lightweight steels with induced plasticity (so-called L-IP), also have high aluminum content. These steels are the lightweight versions of the TWIP steels and are therefore of high interest to the automotive industry. However, the grades existing today have too low yield strength. The yield strength can be improved by alloying and/or by precipitation hardening. Both these techniques require detailed insight on the thermodynamic properties of the alloy system in question. In this work, a thermodynamic reassessment of the entire Al-Fe-Mn system has been performed as a first step to describe the Fe-Al-C-Mn system, the core system for L-IP steels. All available experimental information has been taken into consideration and a set of data has been selected to be used in the optimization. The new thermodynamic description is based on the most recent thermodynamic descriptions of the constituent binaries and reproduces the experimental information in a satisfactory manner. This description, as opposed to previous descriptions has been optimized to fit experimental results in both the aluminum-rich part and the iron-manganese rich part of the system.

The microstructural features required to optimize both the strength and ductility of copper are investigated by examining the as-cast pure Cu and Cu-(1.0e3.0)Fe-0.5Co and Cu-1.5Fe-0.1Sn (wt %) alloys. Uniaxial tensile tests show that (Fe, Co)- or (Fe, Sn)-doping improves both the strength and ductility of pure copper. The microstructure evolution with Fe, Co, or Sn doping is characterized by using optical and scanning and transmission electron microscopies. The effects of Fe, Co, and Sn doping on the microstructure clearly show that (i) iron-rich nanoparticles are dispersed inside the grains. The spherical nanoparticles grow in size with increasing Fe content, and when the Fe content exceeds 2.0 wt %, the particles transition into a petal-like morphology. (ii) The microstructure of the alloys (grain size and morphology) is notably influenced by the Fe and Co contents, and the grain size is reduced from an average of 603 mu m in pure Cu to an average of 26 mm in the Cu-3.0Fe-0.5Co alloy. (iii) The addition of 1.5wt % Fe and 0.1wt % Sn dramatically reduces the grain size to an average of 42 mu m, and this reduction is correlated with the appearance of smaller spherical iron-rich nanoparticles. The evolution mechanisms of the iron-rich nanoparticles and grain structure under the alloying effect are discussed.

This paper presents a brief review and some new results of the thermodynamic calculation and kinetic simulation related to the micro-modelling of solidification processes by using ThermoCalc and DICTRA. Emphasis is given on a partial-equilibrium approach developed as a modification of the Scheil-Gulliver non-equilibrium approximation to treat effectively the back diffusion of fast diffusing elements in solid phases. We start with a DICTRA simulation and successfully explain why the P peak drifts away from those of Mn and Si in the experimentally observed composition profiles of a carbon steel. Then, for this steel and other 16 carbon and low alloy steels, we perform both non-equilibrium and partial-equilibrium solidification calculations and demonstrate the significant improvement of the partial-equilibrium approach by comparing the freezing range results with those from either DICTRA simulations or experimental observations.

The latest development of a thermodynamic database is demonstrated with application examples related to the steelmaking process and steel property predictions. The database, TCOX, has comprehensive descriptions of the solution phases using ionic models. More specifically, applications involving sulphur and oxygen, separately as well as combined, are presented and compared with relevant multi-component experimental information found in the literature. The over-all agreement is good.

This paper demonstrates the use of a new model consisting of a genetic algorithm in combination with thermodynamic calculations and analytical process models to minimize the processing time during a vacuum degassing treatment of liquid steel. The model sets multiple simultaneous targets for final S, N, O, Si and Al levels and uses the total slag mass, the slag composition, the steel composition and the start temperature as optimization variables. The predicted optimal conditions agree well with industrial practice. For those conditions leading to the shortest process time the target compositions for S, N and O are reached almost simultaneously.

The Fe-Mn-S system is of great importance for the steelmaking process. As part of the work for the development of a large self-consistent thermodynamic database for such applications, the aim of the present assessment was to obtain a thermodynamic description of the Fe-Mn-S system using the ionic two-sublattice model for the liquid. Previous compatible descriptions of all unaries as well as two of the constituent binaries, Fe-Mn and Fe-S, were accepted. The third binary, Mn-S, was assessed in this work. Ternary parameters were optimised for the liquid, the alabandite, the pyrrhotite and the pyrite phases. Calculations using the present description agree well with experimental data.

In an effort to describe sulphur in steels, particularly in steelmaking, a thermodynamic description of the Fe-Ca-O-S and its subsystems is presented. The ionic two-sublattice liquid model has been used to describe all liquids. For the Fe-O-S part of the system the calculations reproduce available experimental data well. Experimental data is scarce in the Ca-O-S system and in the quaternary and we to trust that the system is well reproduced by extrapolation by previously assessed sub-systems.

Results of first-principles (FP) total energy calculations for 32 different configurations of the mu phase in the binary system Nb-Ni are used in the compound energy formalism (CEF) to model finite-temperature thermodynamic properties. A comparison with Cluster Expansion Hamiltonian-Cluster Variation Method (CEH-CVM) calculations indicates that the CEF describes temperature-dependent site occupancies as well as the CEH-CVM within the temperature range of interest for applications. This suggests that the Bragg-Williams-Gorsky approximation (BWGA) used in the CEF is sufficient to describe site occupancies and thermodynamics of the mu phase. A phase diagram is calculated using the mu phase description derived in the present work together with a previous Calphad description for the other phases of this system. The FP-CEF approach significantly improves the description of the thermodynamic properties as a function of composition compared to the Calphad procedure generally used up to now.

A coupling between computational fluid dynamics (CFD) and thermodynamics has recently been done. In the current model improvement, a more realistic model was developed, where the numbers of gas species and slag phases were increased. For each simulation the transport of momentum, energy and mass of species as well as the thermodynamic equilibrium in each cell containing at least two phases was treated. Read how this calculation procedure can handle dynamic simulations of interaction between an oxygen gas jet, a melt and a slag. How is the agreement between the predicted rate of decarburization and experimental data? Which findings were achieved from the simulation of four cases varying the temperature, the dissolved carbon content and the dissolved oxygen content?

Thermodynamic databases describe the Gibbs energy of each phase in a given system, as a function of temperature, pressure and its constitution taking into account various properties like crystal structure, solubility of different element on different types of sites, magnetism, etc. The energetics from first principles calculations, crystallographic information and atomistic modelling results can be incorporated into the parametric Gibbs energies models in order to construct thermodynamic databases, being, however, experimental data obtained, e.g. from calorimetry, emf, thermal analyse, metallography, etc. which provides enthalpies, chemical potentials, phase boundary, etc. the basic type of information used. These thermodynamic databases can be an intermediate level between atomistic and macroscopic modelling, bridging scales and theoretical and empirical modelling. Thermodynamic databases provide consistent and accurate information for understanding the macroscopic behaviour of processes and transformations in real materials. This is essential to make realistic modelling and simulations for improving the production as well as teaching of phenomena which are too complex to be modelled directly from a pure theoretical approach. The majority of the phases present in materials for technical applications are solution phases, being its modelling the most complex part of the development of databases. In this paper, we show some features of the making of these databases, as well as the feedback obtained from their different applications and how that results in improvements.

AlxMoNbTaTiV alloys are designed according to the binary phase diagrams and calculation of phase diagrams (CALPHAD). The phase formation and elemental segregation are predicted using the CALPHAD method. The effects of Al on microstructures, and mechnical properties of AlxMoNbTaTiV are investigated, which verified the results of calculated dendritic microstructure and BCC structure. The strengthening mechanism is proved as the result of chemical and electrical interactions. The serration behavior in the compression test at 25 degrees C, 500 degrees C, 700 degrees C, 900 degrees C is discussed. At room temperature, addition of Al enhanced the dynamic aging effects in low concentration. The effects became weaker with the increase of Al content. At 500 degrees C, higher Al concentration leads to stronger serrated flow.

Here, the erythritol-xylitol binary system was thermodynamically optimized based on available experimental phase equilibrium data, to explore compositions suitable as phase change materials (PCMs) for thermal energy storage (TES). A previous experimental study revealed that erythritol-xylitol was a partially isomorphous system with a eutectic. In the thermodynamic evaluation, the CALPHAD method was employed coupling the phase diagram and thermodynamic property information. There, both unary and binary systems’ experimental data were taken into account, and all phases were described using the substitutional solution model. Finally, a self-consistent thermodynamic description for the erythritol-xylitol system was achieved. The calculated eutectic point is at 76.7 °C and 26.8 mol% erythritol, agreeing well with the experimental data. The calculated phase diagram better-verifies the systems’ solidus and the solvus, disclosing the stable phase relations. Based on the Gibbs energy minimization, phase diagrams can be predicted for the binary and higher order systems, provided the component subsystems are thermodynamically assessed beforehand. In conclusion, to move forward beyond e.g. non-isomorphous simple eutectic systems, methods using Gibbs free energy minimization from a fundamental point-of-view such as CALPHAD are essential.

An attempt was made to correct the quasichemical model to avoid predictions of negative configurational entropy when long-range order is impossible as in liquids. The attempt was successful for negative interaction energies but for positive interaction energies the simplest possible variant of the model predicted two narrow miscibility gaps. It was not possible to improve this by adding regular solution parameters. However, using the next possible variant of the correction term these difficulties may be avoided.

Controlling the composition of ternary III-V semiconductor nanowires is of high technological importance and the current theoretical understanding is so far limited. We derive a model for the kinetically limited composition of metal-particle-seeded, ternary nanowires. The model is based on the diffusion controlled growth rate of supercritical nuclei. Applying this model to gold-seeded and self-seeded growth of InxGa1-xAs we are able to explain the experimentally observed features related to nanowire compositions, including the attainability of compositions within the miscibility gap. By directly comparing with experiments we find that 2% arsenic in the alloy particle during self-seeded growth of InGaAs nanowires is a realistic assumption.

We are living in a world of steel. Although there are a lot of other material in use, our most used material is steel. From building industry to transportation and even mother industries like mining, we use steel in different grades and amounts. There is always need for different grades of steel, and there is always interest in better properties and lower costs. Fe-Mn-Al-C steel group is one of the grades of steel is from the TWIP family. Beside its interesting mechanical properties, its corrosion resistance and cryogenic properties makes it very desirable to substitute more expensive current classes of the steel used in the industry. The automobile industry is also looking forward to implement this family of the steel in their products. This group of steel based on their chemical content can created a carbide ordered phase called κ which is one of the reasons of its interesting mechanical properties beside the TWIP properties. While κ may give more hardness due to precipitation hardening, it will make the steel brittle. Thus we need an understanding of the phase diagram of this group of the steels in order to choose our material and process accordingly. Phase diagrams are material engineers’ road maps for the processes and material choice as the initial steps, since we can predict the processes results and stable phases based on the equilibrium state from the diagrams. As the number of components gets more than three the phase diagram calculations and determination gets harder. For the ternary alloying systems we can only show sections of the phase diagrams as isothermal sections, or consider an element constant and depict the diagram as a “binary” system for the other two alloying elements at the other element concentration. In this work, we tried to experiment on the experimental data for equilibrium phases of Fe-Mn-Al-C alloying system based on the Equilibrated Alloys for alloys containing 20, 30 and 40 weight percent Manganese. The results were compared to the current database of the Thermo-Calc software for this family and we found some inconsistencies between the experimental data and the calculations which shows that the calculated results for this alloying system with its high Mn-content, is not reliable and that the thermodynamic descriptions must be adjusted.

In view of the extensive use of Cr as a grain growth inhibitor in WC-Co cemented carbides this thesis comprises a combined experimental and ab initio study of a number of critical issues pertaining to phase equilibria of the subsystems to the W-Co-Cr-C system.

To be able to predict which of the stable Cr-carbides forms above the solubility limits of Cr and C in the fcc phase the respective solubility of Co in Cr₂₃C₆, Cr₇C₃ and Cr₃C₂ are investigated experimentally. Furthermore, the site occupancies of Co in Cr₇C₃ are investigated by neutron diffraction as a first step towards a more realistic model for this phase.

The energetics of the ternary intermetallic R-phase and the unstable Co₃C₂ end-member compound are investigated by density functional theory. For Co₃C₂, the quasi-harmonic vibrational Gibbs energy is also calculated.

By subsequent CALPHAD assessment an improved thermodynamic description is developed and its agreement with experiment is investigated. The resulting thermodynamic description allows for improved accuracy in predicting the formation of carbide phases as a function of C content and Co/Cr ratio as well as liquidus and solidus temperatures.

Remaining issues may be attributed to the thermodynamic description of the W-Cr-C system and the stability of the Cr-based carbides in the Cr-C system. In the case of the Cr-based carbides, severe experimental scatter prevents an accurate determination of the stability of either of them. As a first attempt to resolve the situation, a state of the art ab initio approach is applied to calculate the finite temperature thermodynamic properties of Cr₃C₂, benchmarked with reported heat capacity and relative thermal expansion measurements.

The research presented in this thesis explores the structure, alloying and stability of the carbides in the Co-Cr-C system by using the CALPHAD approach supported by experimental measurements and ab initio calculations.

WC-Co based cemented carbide materials consist of hard WC grains and a ductile Co-based binder. Cr is usually added to cemented carbides for the purpose of reducing grain growth during sintering or to increase corrosion resistance. The Co-Cr-C system is highly relevant for Cr-containing cemented carbides. For example, it determines the allowable C-content interval in the quaternary W-Co-Cr-C system with respect to the formation of unwanted Cr-based carbides.

The CALPHAD method is based on critical assessment of model parameters constituting phenomenological expressions of the Gibbs energy and its derivatives, e.g. enthalpy, entropy and heat capacity. A thermodynamic assessment based on the CALPHAD method relies heavily on the available experimental or ab initio calculated information.

The crystal structures of Cr₂₃C₆ and Cr₃C₂ have previously been accurately determined. However, various authors reported different crystal structures for the Cr₇C₃ carbide. A combined experimental and ab initio investigation of the crystal structure of Cr₇C₃ and M₇C₃ (M=Co,Cr) was thus performed. Furthermore, the stabilities of all end-member compounds of the mixed carbide at 0 K were

determined by ab initio calculations.

Available experimental information on phase equilibria with carbides in the Co-Cr-C system was found to be insufficient to describe all the features in the phase diagram. Therefore, the liquid + M₂₃C₆ + M₇C₃ and M₇C₃ + M₃C₂ + graphite equilibria (M=Co,Cr) were investigated experimentally with focus on the solubility of Co in the M₂₃C₆, M₇C₃ and M₃C₂ carbides.

Finally, a reassessment of the thermodynamic description of the Co-Cr-C system was performed, which resulted in a set of thermodynamic models and parameters that accurately describe all of the considered information.

Applied experimental methods and theoretical models are explained throughout the thesis. Important examples from the appended papers are also given.

The present work aims at assessing the W-Co-Cr system with focus on including the ternary R-phase in the thermodynamic description. Enthalpies of formation at 0 K of all considered R-phase end-members are calculated using density functional theory and used in the assessment due to the scarceness of the experimental information. The resulting assessment is verified by comparing with recent experimental data in the W-Co-Cr-C quaternary system.

Chromium is one of the most well-known WC grain growth inhibitors in cemented carbides. It is thus vital to understand and to be able to thermodynamically model the prevailing phase equilibria in the WC-Co-Cr system. To do this it is important that the lower order systems, such as the Co-Cr-C system, are correctly described. Previous investigations have shown that the M₇C₃ (M=Cr,Co,W) phase is the first carbide to form when Cr is added in excess to the WC+fcc-Co/liquid+graphite phase field. However, the exact structure of this phase has not been investigated and there are many proposed structures already for the binary Cr₇C₃ carbide, ranging from trigonal, via hexagonal to orthorhombic symmetry. Recent investigations show that the hexagonal structure belonging to the P6₃mc space group is the stable structure at 0 K. In the present study the binary Cr₇C₃ carbide and a mixed M₇C₃ carbide are investigated. The structures of both carbides and preferential positions for Co atoms in the mixed carbide are determined by XRD measurements in combination with ab initio calculations and Rietveld refinement.

A reassessment of the thermodynamic description of the Co–Cr–C system was performed to take into account recent experimental information on the solubility of Co in Cr-based carbides. Density Functional Theory (DFT) calculations coupled with phonon calculations were performed to calculate the heat capacity and Gibbs energies of formation of stable and metastable carbides in the Co–Cr–C system as a function of temperature, within the limits of the Harmonic or the Quasi Harmonic Approximation. Resulting Gibbs energies were compared with earlier experimental studies and assessments, where calculated values for Cr₂₃C₆, Cr₇C₃ and Cr₃C₂ from the present work were seen to fall within the experimental scatter. The calculated heat capacity and Gibbs energy of formation as a function of temperature for the metastable Co₃C₂ compound together with recent experimental information was used in the evaluation of the thermodynamic parameters. As a result, the new and improved thermodynamic description accounts for the solubility of Co in M₃C₂ in contrast to previous descriptions, where this was neglected due to a complete lack of experimental information. Furthermore, a better representation of previously reported liquidus temperatures was achieved, without increasing the number of parameters in the liquid phase. Other relevant features of the phase diagram and thermochemical properties were also well represented.

Thermodynamic calculations based on the CALPHAD method are nowadays often applied in the design of new materials due to increasing demands on shorter lead times for development. However, such calculations rely heavily on the assessed thermodynamic descriptions, which in turn rely on the amount and quality of available experimental data, especially for binary and ternary sub-systems. The ternary Co-Cr-C system is an extremely important subsystem to, e.g., multi-component cemented carbide grades, such as W-Co-Cr-M-C (M = Ti,Ta,Nb,V,Zr,Hf), as well as Cr-containing Co-base alloys. In the case of the Co-Cr-C system, there is a lack of reliable data on the solubility of Co in Cr-carbides. Therefore, the present work concerns an experimental study of the solubility of Co in all three of the Cr-based carbides, i.e., Cr23C6, Cr7C3, and Cr3C2. This was done by synthesizing appropriate samples in the M7C3+M23C6+liquid and M7C3+M3C2+graphite three-phase fields. The results show that a recent thermodynamic description of the Co-Cr-C system is unable to reproduce the experimentally determined solubilities. Therefore, the present study provides important input for future alloy development and improvement of the thermodynamic description of the Co-Cr-C system.

Tungsten carbide and cobalt have always been, and still are, the foundation of cemented carbides. Modem grades include several other alloying elements, apart from just WC-Co, added for several different purposes. For example, by adding chromium it is possible to produce extremely fine grained grades compared to straight WC-Co grades and thus the freedom to tailor the properties of the material is increased. By applying thermodynamic calculations it is possible to design the material and also avoid some of the costly trial-and-error procedures. However, there is also a need for experimental verification in order to have confidence in the predictive calculations. The present work concerns the application of thermodynamic calculations to some relevant compositions together with experimental verification in the WC-Co-Cr system. Special focus is given to the limiting conditions for precipitation of unwanted phases with regards to the Cr-content in the binder, C-content and melting temperatures. No regard is taken to the presumable Cr-solubility in the WC phase itself.

The elastic and thermodynamic properties of NiB, Ni2B, Ni3B, orthorhombic Ni4B3(O-Ni4B3), monoclinic Ni4B3(M-Ni4B3), and Ni23B6, are calculated via first-principles method for the Ni-B system. The ground state energies, the full sets of elastic constants and the associated macroscopic elastic parameters of these Ni-B alloys are computed for the first time. Taking contributions from lattice vibrations and thermally excited electrons into account, thermodynamic properties at finite temperatures are then predicted. In addition, we measure the molar heat capacity at constant pressure for NiB and compare the results with the theoretical predictions. Various calculations demonstrate that the first-principles calculation can be used to clarify the diverse experimental data, and provide reliable thermodynamic data. (C) 2010 Elsevier Ltd. All rights reserved.

Thermodynamic stability of Cu2S sulfide in low carbon steels has been investigated using a CALPHAD type thermodynamic calculation method. Thermodynamic properties of the Cu-S binary and Fe-Cu-S ternary systems were critically assessed. By combining the newly assessed thermodynamic parameters to an existing thermodynamic database for steels, a thermodynamic description for low carbon steels involving sulfur and Cu could be obtained and be used to calculate phase equilibria and thermodynamic stability of precipitating phases such as AlN, MnS, and Cu2S. It was predicted that the Cu2S sulfide often observed in low carbon steels is actually a thermodynamically unstable phase and can precipitate when thermodynamic equilibrium state is not reached during steel making processes. Probable reasons and conditions for the formation of this unstable phase are discussed.

Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475 degrees C embrittlement resulting from the phase decomposition in concentrated Fe-Cr alloys. With the aim of revealing the mechanism behind this prosperous phenomenon, here we investigate the effect of Al on the interfacial energy and formation energy of Fe-Cr solid solutions. The interface between the decomposed Fe-rich alpha and Cr-rich alpha' phases carries a positive excess energy, which is of significant importance on determining the process of phase separation. Using ab initio alloy theory, we show that for the alpha-Fe70Cr20Al10/alpha'-Fe100-x-yCryAlx (0 <= x <= 10, 55 <= y <= 80) interface, the Al content (x) barely changes the interfacial energy. However, for the alpha-Fe100-x-yCryAlx/alpha'-Fe10Cr90 (0 <= x <= 10, 0 <= y <= 25) interface, the interfacial energy increases with Al content due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying. Our ab initio results are supported by CALPHAD calculations, and suggest that the beneficial effect of Al on ferritic steels is mainly due to its thermodynamical effect on the alpha' phase.

The phase equilibria and thermochemical properties of the Co-Cr system was re-assessed using the Calphad method with the aid of ab initio calculations. Four solution phases, liquid, bcc, fcc and hcp phases are modeled using the substitutional solution model. The sigma phase, as an intermetallic compound with certain homogeneous composition range, is modeled by a full five sublattices partitioned CEF model. The formation enthalpies of all end-members are obtained from ab initio calculations. The calculated results fit the experimental data reasonably well which indicates that this ab initio aided model is a proper description for the sigma phase.

The thermodynamic functions of the pure Co were assessed using CALPAHD method for the third generation thermodynamic databases. To model the magnetic properties of the cobalt, a two-state magnetic model was accounted for the fcc phase. Calculated results were compared with the experimental information and a good fit to the experimental data was achieved.

The world is facing serious challenges regarding environmental issues. Carbon dioxide levels increase every day. In an attempt to decrease carbon emissions the automotive industry searches for lighter, stronger materials. TWinning Induced Plasticity (TWIP) steels show an impressive combination of strength and ductility. The possibility of adding high amounts of aluminum that decreases the density while maintaining the mechanical properties makes this type of steel very interesting for use in automotive applications. The only thing keeping the TWIP steels from being used in the automotive industry is that the Yield Strength (YS) is too low. The TWIP steels usually have a YS around 400 MPa. For them to be useful in automotive applications YS around 600-700 MPa is necessary. One of the most promising ways of improving the YS is by precipitation hardening. This work has been performed within a European Research Fund for Coal and Steel (RFCS) project called Precipitation in High Manganese steels (PrecHiMn). As the name of the project suggests, the goal of this project has been to study the precipitation in TWIP steels. The precipitation hardening is achieved through the addition of carbide and carbonitride formers such as Nb, Ti and V. In order to build advanced models to simulate precipitation it is important to have a good understanding of the thermodynamics of systems related to TWIP steels.

The goal of this work has been to study the thermodynamic properties of systems related to TWIP steels, more specifically the system forming the matrix phases of TWIP steels. Therefore the Al-C-Fe-Mn system has been studied as well as the Al-Ti-V system. Complete thermodynamic descriptions that reproduce the experimental data well have been produced including descriptions of order-disorder transformations.

TWIP (TWinning Induced Plasticity) steels are attracting a lot of attention due to their combination of strength and ductility. In a previous work [1] (B. Lindahl, M. Selleby, Calphad 43 (2013) 86-93) a thermodynamic assessment of the Al-Fe-Mn system, which forms the basis of TWIP steels, was presented. The previous assessment treated the A2/B2 order-disorder transformation in the bcc phase using a two-sublattice model. In the present work a four-sublattice model has been used in order to also be able to describe the transition into the ordered DO3 compound that occurs at lower temperatures. pair interaction energies for the Fe-Mn system are evaluated which prove crucial to the extrapolations into the Al-Fe-Mn system. Along with this various aspects of modeling chemical ordering using the Calphad approach are discussed. Equations for determining the ternary compound energies from binary pair interactions energies are presented and equations for determining the parameter values from the ordered parameters are derived.

Many phenomena determining the microstructure of a tool steel and consequently the properties of the material, are governed by multicomponent diffusion. The diffusion driven reactions that take place during, for example, tempering of a hot-work tool steel or when the microstructure develops during hot isostatic pressing of cold-work tool steel, are dependent on the types and amounts of alloying elements. In order for computational methods to be usable, these alloying effects need to be understood and incorporated in the models.

In this work the influence of some typical tool steel alloying elements on the coarsening behavior of precipitates is investigated. Experimental coarsening studies are performed and the impact of the diffusion mobility descriptions and the thermodynamic descriptions are investigated by means of DICTRA coarsening calculations. The kinetic descriptions for diffusion in the body centered-cubic phase in the case of the chromium-iron-vanadium system and the chromium-iron-molybdenum system are improved by assessments of diffusion mobility parameters, and are shown to have a large impact on the calculated coarsening rate for vanadium-rich and molybdenum-rich precipitates.

The effect of cobalt is examined by a coarsening experiment for vanandiumrich carbides and by a diffusion couple experiment for the investigation of the vanadium interdiffusion. The presence of cobalt is experimentally shown to have retarding effect on the coarsening rate of the carbides, but not on the vanadium diffusion.

The coarsening rate of nitrogen-rich precipitates is compared to the coarsening rate of carbon-rich precipitates, and a lower coarsening rate for nitrides compared to carbides can be confirmed. Correlation between coarsening calculations and experiments is obtained suggesting that the thermodynamic description of the two systems is the underlaying reason for the different coarsening rates.

Further, calculations utilizing the DICTRA software are combined with experimental investigations in order to study the possibility to apply computational methods for compound material development and explore application areas for high nitrogen alloyed tool steels produced by powder metallurgy.

The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-gamma, and tetragonal-sigma phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-gamma phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings.

 The molar volume and thermal expansion of selected metallic elements with the bcc, fcc and hcp structures were studied by means of the Calphad approach. Experimental data were critically assessed, and model parameters were obtained yielding reasonable descriptions of all experimental data on molar volume and thermal expansion. The descriptions are valid between room temperature and the melting points at atmospheric pressure.

In the past three decades, nanostructured (NS) and ultrafine-microstructure (UFM) materials have received extensive attention due to their excellent mechanical properties such as high strength. However, preparing low-cost and bulk NS and UFM materials remains to be a challenge, which limits their industrial applications. Here, we report a new strategy to prepare bulk UFM alloys via the direct solidification of high-entropy alloys (HEAs). As a proof of concept, we designed AlCoCrxFeNi (1.8 <= x <= 2.0) HEAs and achieved a complete UFM in bulk materials. The compositional requirements for obtaining the formation of the UFM are highly demanding, necessitating the coupling of near eutectic alloy composition and the high temperature decomposition of supersaturated primary and secondary phases. Our strategy provides a low-cost and highly efficient method to prepare bulk UFM alloys, with great potential to accelerate the engineering application of these materials.

Alumino-silicate systems are of great interest for materials scientists and geochemists. Thermodynamic knowledge of these systems is useful in steel and ceramic industries, and for understanding geochemical processes. A popular and efficient approach used to obtain a self-consistent thermodynamic dataset is called CALPHAD. It couples phase diagram information and thermochemical data with the assistance of computer models. The CALPHAD approach is applied in this work to the thermodynamic modelling and assessments of the CaO-Al₂O₃-SiO₂, MgO-Al₂O₃-SiO₂ and Y₂O₃-Al₂O₃-SiO₂ systems and their subsystems. The compound energy formalism is used for all the solution phases including mullite, YAM, spinel and halite. In particular, the ionic two sub-lattice model is applied to the liquid solution phase.  Based both on recent experimental investigations and theoretical studies, a new species, AlO₂^-1, is introduced to model liquid Al₂O₃. Thus, the liquid model corresponding for a ternary Al₂O₃-SiO₂-M₂O_m system has the formula (Al⁺³,M^+m)_P (AlO₂^-1,O^-2, SiO₄^-4,SiO₂⁰)_Q , where M^+m stands for Ca⁺², Mg⁺² or Y⁺³. This model overcomes the long-existing difficulty of suppressing the liquid miscibility gap in the ternary systems originating from the Al₂O₃-free side during the assessments.  All the available and updated experimental information in these systems are critically evaluated and finally a self-consistent thermodynamic dataset is achieved. The database can be used along with software for Gibbs energy minimization to calculate any type of phase diagram and all thermodynamic properties. Various phase diagrams, isothermal and isoplethal sections, and thermochemical properties are presented and compared with the experimental data. Model calculated site fractions of species are also discussed. All optimization processes and calculations are performed using the Thermo-Calc software package.

The CaO-Al2O3-SiO2 system has been assessed with the CALP-HAD technique, based on recent assessments of its binary systems. A new species AlO2-1 was introduced for modeling liquid Al2O3. The ternary liquid phase was described using the ionic two-sublattice model as (Al+3, Ca+2)(p) (AlO2-1, O-2, SiO4-4 SiO20)(Q). The available experimental data were critically examined and a self-consistent set of thermodynamic descriptions was obtained. Various phase diagrams and property diagrams, including isothermal sections, isoactivity lines, and a projection of the liquidus surface, are presented. Information on viscosity seems to support the use of the AlO2-1 species.

The technically important quasi-ternary section Si3N4-AlN-Al2O3-SiO2 of the Si-Al-O-N system has been then-nodynamically reassessed. Improved descriptions for the Gibbs energies of the beta and O'-sialon phases are applied. Different modelings according to the different behaviors of the reciprocal system within the beta-sialon phase are discussed. The liquid phase is modeled with a new formula based on the ionic twosublattice model. The thermodynamic properties of the sialon phases are discussed and various phase diagrams are presented. The self-consistent thermodynamic dataset is useful for the computer simulation of sialon synthesis. Some examples of such applications are illustrated.